An isometric virus-like particle (VLP) approximately 20nm in diameter has been isolated from three strains of Neurospora crassa: a wild type (OR23-1A), "poky" (1578), and P-147. Based on co- sedimentation with known standards, the sedimentation constant for the particle is about 86S. The particles from the three strains appear morphologically identical, but no immunological tests of relatedness have yet been conducted. Purification of the particles involves precipitation with polyethylene glycol followed by differential centrifugation and separation on sucrose gradients. Absorption spectra are consistent with the particles being a nucleo-protein containing 5-15% nucleic acid. The particles when purified appear to be unstable since following dialysis and sedimentation through a second sucrose gradient, their sedimentation constant is lowered (about 73S). Concomitant with this change the UV absorption spectrum approaches that of a protein. Purification in the presence of EDTA reduces the instability of the particles. The quantity of VLP (based on UV absorption) extractable from wild type is markedly less than that from "poky" and P-147. It is not yet clear whether the higher titers of VLP's in "poky" and P-147 are causally related to any physiological parameter such as the slow growth characteristic of those strains. During the coming year, our efforts will be directed at determining the nucleic acid contained in the 20nm particle and attempting to correlate quantities of VLP's with slow growth. Methods for the purification of a second VLP in the P147 strain will be developed. In addition, a VLP carried by the unknown (un) mutant of Neurospora is being investigated.